Safety system for motor operated machines



April 1969 J. CONNELL ET AL 3,436,553

SAFETY SYSTEM FOR MOTOR OPERATED MACHINES Filed Dec. 4, 1967 Sheet of 2 FIG.

FL I 9N (SPF we I84 M JAME CONNELL.

THOM G. COMERFORD INVENTORS FMZZMW ATTORNEYS April 1 9 J. CONNELL ET AL SAFETY SYSTEM FOR MOTOR OPERATED MACHINES Sheet Filed Dec. 4; 1967 FIG. 2

JAMES CONNELL THOMAS G. COMERFORD INVENTORS TO MONITOR POINTS ATTORNEYS United States Patent 3,436,558 SAFETY SYSTEM FOR MOTOR OPERATED MACHINES James Connell, Livonia, and Thomas G. Comerford, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Dec. 4, 1967, Ser. No. 687,798 Int. Cl. H021) 1/24; I-IOlh 19/64, 33/52 US. Cl. 307113 5 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention This invention relates to a safety control device for a motor operated machine, and is described in particular in connection with a control device for insuring safety in setting up and threading the work stock into a roll feed punch press.

Description of the prior art As might well as imagined, machines requiring a human operator to feed a workpiece between a reciprocating member movable into mating engagement with a fixed member, are inherently dangerous. Through carelessness or inadvertence, the operator may at times have his hands in the press chamber and under the reciprocating member as it is moving downward, with the result that injuries are not uncommon. The prior art has provided a number of safeguards; the inherent thrust of most of these is to use mechanical guards and/or to require the operator to use both hands to operate the various controls so that there is no danger that his hands will be crushed under the mating members. In theory, the twohand safety system should have solved the problem. However, human nature being what it is, the tendency has been for the operator to try to defeat the safety device by resorting to subterfuge, such as, for example, holding a switch permanently closed by a weight or in some other spurious manner, so as to free one hand for unauthorized activity.

The present invention also utilizes the same general method of attack, that is, it is a control device requiring the full utilization of both hands of the operator, so that they cannot be under the mating mechanism. There is no known prior art arrangement that suggests the solution presented here.

SUMMARY OF THE INVENTION The safety device of the invention is illustrated by its application to a roll feed punch press in which a stock clamping means is utilized to hold the work stock in a fixed position during the setup operation. The switch controlling the stock clamping means is electrically interlocked with other switching components, arranged in spaced relationship to each other, so that the press opera- 'ice tor must physically use both hands to operate the switches simultaneously in order to secure requisite electric power to accomplish intermittent displacement or inching of the reciprocating member.

Accordingly, it is an object of this invention to provide a fool-proof safety arrangement for a motor driven machine wherein a plurality of switches are provided and arranged in spaced relationship so that the operator is required to use both hands in order to energize the reciprocating member of the machine.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, can best be understood by reference to the following description considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic front elevational view of a roll feed punch press showing the stock clamping means, and cooperating control switching components arranged in accordance with the invention;

FIG. 2 is an electrical schematic showing the control circuitry in accordance with the invention;

FIGS. 3A and 3B are electrical schematics showing the' clamp switch in the unlocked and locked positions, respectively.

The invention will be described in its application to a roll feed punch press of a type commonly found in industry. Referring now to FIG. 1, a supply roll, containing the metal to be formed, is indicated at 10. The metal stock 12 is fed from the roll 10, over the table 14, and is passed between feed rollers 16, 18 and then into the press chamber indicated generally at 20. A reciprocating member or press platen 22, supporting a mating die 24 is arranged for rectilinear displacement in the vertical direction as indicated by the arrow at 26. A press bed indicated at 28, supports a stationary die shown at 30, and as may be seen from a study of this schematic, the stock 12 is fed over the stationary die 30.

The operator has various buttons to press for electrically actuating the various switches in order to connect an electric motor, so as to actuate the press plate 22. On a support (unnumbered) to the right of the operator, there is located a Mode Control Switch indicated generally at 32; this switch enables the operator to select the mode of operation: Continuous Run (Cont), Single Stroke (8.8.), or Inch. (A discussion of these various modes will be given later as this description proceeds.) Just below the Mode Control Switch 32, there are located three buttons indicated generally at 34, 36, 38, and denominated Start Switch, Inch Switch, and Stop Switch, respectively. To the operators left there are located the Air Clamp Switch indicated generally at 40, and the Jog Switch indicated generally at 42.

The punch press is provided with a stock clamping means indicated generally at 44. The stock clamping means 44 is pneumatically controlled for vertical displace ment by means of an air cylinder 46, controlled by means of a solenoid actuated pneumatic valve 48, which supplies air under pressure to the air cylinder 46 by means of the penumatic line 50. The solenoid for actuating the valve 48 will be presently identified and described in the description of the circuitry shown in FIG. 2.

Referring now to FIG. 2, the press platen 22 is suitably coupled to an electric motor 52, which is energized from a 440-volt 3-phase 60-cycle supply. Line contacts from the supply to the electric motor 52 are indicated in each phase at 54, 56, 58; these contacts are normally open as shown, and are closed upon the actuation of a master magnetic line starter switch, not shown on the drawing.

A motor control relay is indicated symbolically at the motor control relay 60 has four normally open contacts indicated at 62, 64, 66, and 68. Overload thermal coils for the motor 52 are indicated in the line conductors at and 72, respectively. A control relay is indicated at 74; this relay contains two normally open contacts 76, 78 which are arranged in series with the coil of relay 60. As may be observed from a study of the schematic diagram of FIG. 2, the control relay 60 is connected across one phase of the 3-phase 440-volt line.

A step-down transformer is indicated generally at 80. The primary 82 of the transformer is connected across one phase of the 3-phase 440-volt line supply voltage as shown; the transformer secondary, indicated at 84, provides a step-down voltage of volts. One side of the secondary 84 is grounded as indicated at 86; the high side of the transformer secondary includes a fuse at 88. A control relay indicated at 90, has two normally open contacts indicated at 92, 94.

The Mode Select Switch 32 is a multicontact ganged switch having a plurality of cooperating contacts arranged in spaced relationship as indicated at 96, 98; 100, 102; 104, 106; 108, 110; and 112, 114.

The Start Switch 34 is a three-decked switch having contact pairs at 116, 118; 120, 122; and 124, 126. The contacts 96, 98 of the Mode Select Switch 32 and the contacts 116 and 118 of the Start Switch 34 are electrically connected in series with the relay 90, the series combination being across the transformer secondar 84.

The Stop Switch 38 is a two-decked switch having cooperating contacts, indicated at: 128, 132, 134.

As will be readily appreciated from a study of the circuit diagram, the contact points 100, 102 of the Mode Select Switch 32, the normally open contacts (unnumbered) of the Jog Switch 42, contacts 128, 130 of the Stop Switch 38, and an overload thermal coil 136 are connected in series with the control relay 74, the series combination being connected across the secondary 84 of the transformer 80.

A solenoid is indicated symbolically at 138; this solenoid controls the clutch (not shown) for engaging and disengaging the coupling between the motor 52 and the press platen 22. A control relay is indicated at 140; this relay has three normally open contacts indicated at 142, 144, 146. The contacts 142, solenoid coil 138, and contacts 144 are connected in series, the series combination being connected across one phase of the 3-phase 440-volt supply voltage.

A step-down transformer indicated generally at 148 has its primary 150 connected across one phase of the 440- volt supply line, and across the previously recited series combination of contacts 142 and 144- and the solenoid coil 138. The secondary 152 of the transformer 148 provides a step-down voltage of 110 volts for the control circuitry about to be described.

One side of the secondary 152 is grounded at 154; the other side of the transformer secondary 152 includes a fuse 156. A master safety circuit is indicated generally at 158. This circuit is shown as a block diagram merely for the sake of completeness, and forms no part of this invention; the details of the circuitry within the block will therefore be omitted. It will be sufiicient to indicate at this time that the master safety circuit 158 is grounded by line 160 and it includes a terminal strip from which a number of lines are led to the various points about the machine environment to be monitored. The master safety switch 158 is connected with the voltage supplied across the transformer secondary 152 through a 3-pole double throw tumbler switch indicated generally at 162. It should be noted that the switch 162 is shown in two parts here merely for the sake of convenience, although it will be understood that in practice, the switch 162 is a unitary assembly. Two contact points of the 3-pole double throw tumbler switch 162 are connected to one side of the transformer secondary 152 by means of blade elements 164,

4 166 and conductors 168, 170. In the position shown, the switch 162 is in the position for energizing the master safety circuit 158. The switch 162 contains two contact points that will be of particular interest to us in the description to follow, and therefore they will be separately identified now as 172 and 174.

The air clamp switch 40 contains two pairs of spaced contacts indicated at 176, 178; 180, 182. One side of solenoid coil 184, used for actuating the air valve 48 for holding the stock clamp 44, is connected to the grounded side of the transformer secondary 152; the other side being connected to contact 180 of the air clamp switch 40. The contacts (unnumbered) of the Inch Switch 36 are connected to the contact point 178 and to the relay 140.

Completing the description of the electrical circuit of FIG. 2, the control relay 186 includes two normally open contacts indicated at 188 and 190. A contact limit switch indicated at 192 is of the spring return type, normally closed as indicated, and having its contacts connected between contact points 108 and 112 of the Mode Select Switch 32. A guard microswitch 194 has the electrical positions indicated at 196, 198, which points are electrically the same as the contacts 104 and 108, respectively, of the Mode Select Switch 32.

The air clamp switch 40 is shown in two operating positions depicted in FIGS. 3A and 3B. The air clamp switch 40 is utilized with cooperating mechanical and electrical means to provide the requisite electrical connections. With the air clamp switch in the on position shown in FIG. 3A, a knurled ring is rotated to the left, so that the contacts 180, 182 are contiguous and are electrically one. This by definition is the unlocked position. It will be seen from FIG. 3A, that contacts 180 and 182 are electrically common and in series with solenoid 184.

In the position shown in FIG. 3B, the knurled ring is rotated by the operator to the right, so that the air clamp switch is in the elf position with the contacts 180, 182 open as shown. This by definition is the locked position. It should be noted carefully that in both the FIG. 3A and FIG. 3B configuration, electrical connection between the contacts 176 and 178 can still be accomplished by pressure on the bridging member to close the contacts 176 and 178.

ENVIRONMENT OF THE INVENTION The punching operation disclosed in FIG. 1 makes use of a so-called progressive die, which comprises several stations at each of which a punching or forming operation is accomplished with each cycle of the press platen 22, the stock being advanced from one station to the next while the press platen is up, the finished part being cut from the web stock at the last station to drop into a chute leading into a tray. The so-called stations are all located on the stationary die member 30. The usual first operation may be, for example, to punch a locating hole in the stock, corresponding to one piece, this hole being engaged at each subsequent station by a pilot pin as the die members 24, 30 are brought together by the downward movement of the press platen 22. The stock feed rate, which is controlled by the feed rollers 16, 18, must be very accurately adjusted for a particular die during the setup operation of the machine, so that the successive locating holes will be centered within 1.005 inch under the multi-station pilot pins for each cycle of stock advance and die engagement.

During the normal production operation, the rollers 16, 18 feed the stock into the press chamber 20 at a preestablished rate, and they then stop and are automatically disengaged, the pilot pins entering the locating holes in the stock 12. Since the feed rollers 16, 18 are disengaged, the stock 12 is thus free to be precisely positioned by the tapered shanks of the pilot pins, as the die members 24, 30 engage to punch or form the stock. The feed rollers 16, 18 are automatically reengaged, clamping the stock again, before the separation of the die members 24, 30 is effected. Any error in the feed rate greater than 1:.005 inch requires excessive positioning action of the pilot pins, which may result in potentially serious damage to the dies. The setup operation is therefore intended to determine the correct feed rate. In previous practice, this was a hazardous operation. The guard window in front of the pressure chamber was removed and the operator physically moved the stock 12 with his hands as he attempted to correctly align the stock locating holes with the pilot pin. In practice, it was intended that the operator would operate the Jog Switch 42 and the Inch Switch 36, with the fingers of the left and right hands, respectively, so that in theory, his hands would never be within the chamber 20. However, in practice, the actuation of the log Switch would impart sufiicient momentum to the flywheel, so that the press platen member 22 could be slowly advanced or inched downward by means of the inch switch alone, leaving one hand free to enter the chamber 20 to move the stock 12 into position. As we shall see, the present invention prevents such action during the setup operation.

The punch press of FIG. 1 may be operated as previously indicated in any one of three modes of operation: Continuous Run (Cont.), Single Stroke (8.8.), and Inch. The Continuous Run Mode is intended when the machine is operating during production, and the Single Stroke Mode is utilized for any operation requiring a single downward stroke of the press platen member 22, bringing the dies 24 and together just once. The Inch Mode is utilized solely to give the operator of the machine a chance to advance the press platen member 22 in incremental displacements during the set-up operation. During the Continuous Run and Single Stroke operations, the question of safety does not arise, because a guard window seals off the press chamber 20 so that it is not possible for the operator to place his hands within this working chamber. We shall only be concerned here with the Inch Mode of operation, which is the mode that must be selected for the set-up operation.

OPERATION Referring now particularly to FIG. 2, during the set-up operation for the punch press, the operator trips tumbler switch 162 to the OE position. The Mode Selector Switch 32 is advanced to Inch. The internal contacts of Mode Selector Switch during the Inch Mode are electrically as follows:

96-98 Open 100-102 Closed 104106 Closed 108-110 Open 112-114 Open For ready reference, the various contact relays and their contacts are regrouped as follows:

The guard window (not shown in the drawing) in front of the press chamber 120, is opened so that the guard microswitch 194 is in the position shown in FIG. 2, making contact with terminal 196.

The feed rollers 16 and 18 are disengaged and held open by a manually operated lever mechanism (not shown). The stock 12 is fed through the open rollers into the press chamber 20 and above the stationary member 30 of the progressive die to a point just beyond the first die station. The ring on the Air Clamp is rotated to the left, so that electrically, the Air Clamp 40 has the position shown in FIG. 3A. The Air Clamp Switch 40 is then actuated and held closed by a finger of the left hand. This closes contacts 176, 178 and electrically completes the circuit to the solenoid coil 184, causing the air valve 48 to operate the air cylinder 46 and displace the stock clamp 44 vertically downward, holding the stock 12 in position. By tracing through the current from the transformer secondary 152, it will be seen that the transformer secondary voltage is between the conductor 170 and ground. The electrical circuit for the solenoid 184 may be traced from the conductor 170, through Stop Switch 38, contact 196, contacts 104, 106, contact 174, contacts 176, 178, contacts 182., 180, and to ground through the coil 184.

The stock 12 is now being held and the Inch Circuit is enabled. When the Inch Switch 36 is pressed, the relay 140 is energized, the relay 140 completing itself through the Inch Switch 36 and through the Air Clamp Switch 40.

The energization of the relay 140 closes its contacts 142, 144, 146. The closing of contacts 142, 144 energizes the solenoid 138 so that the clutch is engaged. However, the motor cannot turn over because contacts 62, 64, 66 remain open, and it is therefore not energized. The actual turnover of the motor must be accompanied by the closing of the Jog Switch 42. Note in the Inch mode, the contacts and 102 of the Mode Selector Switch are closed. The closing of the Jog Switch 42 completes the circuit for the coil 74, and it is energized, closing its contact 76, 78. The closure of 76, 78 then completes the circuit for the relay 60, and it too is energized, causing its normally open contacts 62, 64, 66 to close, thus energizing the circuit for the electric motor 52. It should be clear from the way in which the operation has just been described that operation of the motor 52 in the Inch Mode requires the closure of three separate switches: The log Switch 42, the Air Clamp Switch 40, and the Inch Switch 36. As may be appreciated best from a study of FIG. 1, this must be accomplished by the operators using two fingers of the left hand to operate the Air Clamp and Jog Switches and one finger of the right hand to selectively close the Inch Switch. With both hands occupied, this eflectively prevents the operators placing his hands within the press chamber 20. Stated succinctly, the press platen 22 will not cycle unless the Jog, Air Clamp, and Inch Switches are simultaneously activated.

The switches are next released and the stock 12 is next advanced manually to the approximate position of the next station. The Jog, Air Clamp, and Inch Switches are again actuated, and the stock 12 is again clamped while the press platen 22 is inched downward until the pilot pin approaches the locating hole in the stock. The stock adjustment and inching operations are continued until the pilot pins exactly enter the locating holes.

The sequence of operations may be repeated for each station to check performance through the full succession of stations. When it appears that the correct alignment has been achieved, an adjustment is made to the machine controls to establish what is expected to be the approximately correct feed rate for the feed rollers 16, 18. The rollers 16, 18 are engaged by a manually operated lever arrangement (not shown on the drawing), while the stock is held in position between the closed die members 24, 30. The engaged feed rollers 16 and 18 now provide clamping action, and accordingly, the stock clamp 40 is now adjusted by manually turning the lock-out ring which surrounds the operating button of the air clamp switch 40. This electrically produces the condition indicated in FIG. 3B, the electrical interlock between the Air Clamp Switch 40 and the Inch Switch being kept intact. The Air Clamp Switch 40 and Jog Switch 42 must still be closed by the left hand, while the Inch Switch 30 is actuated by the right hand to cause the press platen to be displaced. When the point in the operating cycle is reached at which the pilot pin or pins are about to enter the associated locating holes in the stock, a check is made to determine the correctness of the feed rate which occurred during the cycle as established by the tentative adjustment of the machine controls. If a change in feed rate is necessary, the above steps relating to manual adjustment of stock position are repeated, and the feed rate controls are readjusted and another test cycle is carried out. This adjustment sequence is repeated until a satisfactory feed rate is established, after which the Mode Control Switch 32 is set to Continuous Run, the guard window is closed, and the Start Switch 34 is actuated to initiate automatic press operation.

The components which are shown in the circuit of FIG. 2, which have not been referred to, play no dynamic role during the Inch Mode operation, and accordingly are not a part of this invention. For example, control relay 90, Start Switch 34, and Limit Switch 192 are not used during the Inch Mode operation. The Jog Switch 42 is used only during the Inch Mode operation. The limit switch 192 is caused to disconnect by the action of a cam mounted coaxial with the flywheel, for the purpose of providing make-and-break energization of the circuit during single stroke mode of operation.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

We claim:

1. A safety device for a motor operated machine having a reciprocating press platen member movable into mating engagement with a stationary die, the stock to be formed being progressively advanced between the mating members, said safety device comprising:

(a) an electric motor electromechanically coupled to said reciprocating press platen member;

(-b) a source of electrical energy;

(c) control circuit means connected to said electrical source for selectively energizing the electromechanical coupling between said press platen member and said electric motor;

((1) clamping means for rigidly holding the stock in a nonadvance position during the setup operation;

(e) first switching means connected to said electrical source for actuating said clamping means;

(f) second switching means electrically connected to said first switching means and to control circuit means, the actuation of said first switching means enabling said second switching means to actuate said control circuit means; and

(g) third switching means for electrically completing the connecting circuit between said source of electrical energy and said electric motor, any two of said switching means being physically in spaced relationship so as to require the human operator to use both hands for actuation;

whereby the simultaneous actuation of all three switching means is required to provide incremental displacement control for said reciprocating press platen member.

2. A safety device for a motor operated machine having a reciprocating press platen member movable into mating engagement with a stationary die, the stock to be formed being progressively advanced between the mating members, said safety device comprising:

(a) an electric motor electromechanically coupled to said reciprocating press platen member;

(b) a source of electrical energy;

(c) control circuit means connected to said electrical source for selectively energizing the electromechanical coupling between said press platen member and said electric motor;

(d) clamping means for rigidly holding the stock in a nonadvance position during the setup operation, comprising a solenoid actuator, a clamping member coupled to said solenoid actuator, the energization of said solenoid actuator displacing the clamping member into contiguous engagement with the stock;

(e) first switching means comprising two pairs of electrical contacts, the closure of said first pair partially completing the circuit to said solenoid actuator, the closure of the second pair energizing said solenoid actuator;

(f) second switching means electrically connected to said second pair of contacts of said first switching means and to said control circuit means the actuation of said second pair of contacts of said first switching means enabling said second switching means to actuate said control circuit means; and

(g) third switching means for electrically completing the connecting circuit between said source of electrical energy and said electric motor, any two of said switching means being physically in spaced relationship so as to require the human operator to use both hands for actuation;

whereby the simultaneous actuation of all three switching means is required to provide incremental displacement control for said reciprocating press platen member.

3. A safety device for a motor operated machine having a reciprocating press platen member movable into mating engagement with a stationary die, the stock to be formed being progressively advanced between the mating members, said safety device comprising:

(a) an electric motor electromechanically coupled to said reciprocating press platen member;

(b) a source of electrical energy;

(c) control circuit means connected to said electrical source and electrically connected with the electromechanical coupling between said press platen member and said electric motor;

(d) clamping means for rigidly holding the stock in a nonadvance position during the setup operation;

(e) first switching means connected to said electrical source for actuating said clamping means;

(f) control relay means having a pair of normally open contacts connected to said control circuit means, the closure of said normally open contacts energizing the control circuit means to actuate said electromechanical coupling;

(g) second switching means electrically connected to said first switching means and to said control relay means, the actuation of said first switching means enabling said second switching means to energize said control relay;

(h) third switching means for electrically completing the connecting circuit between said source of electrical energy and said electric motor, any two of said switchng means being physically in spaced relationship so as to require the human operator to use both hands for their actuation;

whereby the simultaneous actuation of all three switching means is required to provide incremental displacement control for said reciprocating press platen member.

4. A safety device for a motor operated machine having a reciprocating press platen member movable into mating engagement with a stationary die, the stock to be formed being progressively advanced between the mating members, said safety device comprising:

(a) an electric motor electromechanically coupled to said reciprocating press platen member;

(b) a source of electrical energy;

(c) control circuit means connected to said electrical source for selectively energizing the electromechanical coupling between said press platen member and said electric motor;

(d) clamping means for rigidly holding the stock in a nonadvance position during the setup operation;

(e) first switching means connected to said electrical source for actuating said clamping means;

(f) second switching means electrically connected to said first switching means and to said control circuit means, the actuation of said first switching means enabling said second switching means to actuate said control circuit means;

(g) first electromagnetic control relay means comprising a first coil, and having normally open contacts connected in the circuit between said source of electrical energy and said motor;

(-11) second electromagnetic control relay comprising a second coil and normally open contacts connected in series with said first coil;

(i) third switching means connected in series with said second coil, the actuating of said third switching means successively energizing said second and first electromagnetic control relays, completing the circuit between said source of electrical energy and said electric motor, any two of said switching means being physically in spaced relationship so as to require the human operator to use both hands for actuation;

whereby the simultaneous actuation of all three switching means is required to provide incremental displacement control for said reciprocating press platen member.

5. A safety device for a motor operated machine having a reciprocating press platen member movable into mating engagement with a stationary die, the stock to be formed being progressively advanced between the mating members, said safety device comprising:

(a) an electric motor electromechanically coupled to said reciprocating press platen member;

(b) a source of electrical energy;

(c) control circuit means connected to said electrical source, for selectively energizing the electromechanical coupling between said press platen member and said electric motor;

(d) means for rigidly holding the stock in a nonadvance position during the setup operation, comprising a stock clamping means, a pneumatic valve, and a solenoid actuator, said pneumatic valve being adapted for actuation by said solenoid actuator, and being pneumatically coupled to said stock clamping means, the actuation of said pneumatic valve supplying air under pressure to displace said stock clamping member into contiguous engagement with said stock;

(e) first pair of normally open electrical contacts arranged to be closed to partially complete the electrical path to said solenoid actuator;

(f) first switching means comprising a second pair of normally open contacts connected to said electrical source, the closure of said second contact pair electrically energizing said solenoid actuator;

(g) second switching means electrically connected to said second pair of contacts of said first switching means and to said control circuit means, the actuation of said second pair of contacts of said first switching means enabling said second switching means to actuate said control circuit means; and

(h) third switching means for electrically completing the connecting circuit between said source of electrical energy and the connecting circuit between said source of electrical energy and said electric motor, any two of said switching means being physically spaced far enough apart so as to require the human operator to use both hands for actuation;

whereby the simultaneous actuation of all three switching means is required to provide incremental displacement control for said reciprocating press platen member.

References Cited UNITED STATES PATENTS 2,802,154 8/1957 Bonn et al 307-412 X 2,888,576 5/1959 Giboney et al. 307112 X 3,183,377 5/1965 Holland 307-113 3,311,794 3/1967 Holland et a1. 307113 X 3,315,243 4/1967 Weiberg 317135 X ROBERT K. SCHAEFER, Primary Examiner. T. B. JOIKE, Assistant Examiner.

U.S. Cl. X.R. 

